Circuit breakers at medium voltages usually need to employ current sensing devices or transformers to detect primary current overloads and short-circuits for monitoring and protection. In indoor applications, these may be incorporated in the equipment of which the circuit breaker is a component part On the other hand, in outdoor applications, particularly in rural electrification schemes, where the circuit breaker is often used in overhead line applications, the current sensor or transformer is preferably incorporated as an integral part of the circuit breaker.
This is usually achieved by mounting a current sensor, such as a ring-type current sensor or transformer, coaxial with an insulated conductor or bushing. Typical examples of conventional use are shown in FIGS. 1 and 2. The interrupting device could be typically a vacuum or gas switch.
The design in FIG. 1 usually requires some form of additional liquid or gaseous insulation, such as oil or SF6, to keep the size of the circuit breaker to acceptable levels and also to ensure that the internal components are maintained free of moisture and contamination. A more recent design is depicted generally in FIG. 2. Here, the need for a tank filled with oil or SF6 is removed. The current transformer or sensor is mounted at the side of the switch and electrically in series with it. This example uses a vacuum switch and current transformer encapsulated in solid insulation.
In both cases, however, it is still necessary for the insulation exposed to outside environmental conditions to have additional “creepage” length compared to insulation that is protected from the external environment. Thus, although the typical design illustrated in FIG. 2 does not need liquid or gaseous insulation material to minimise the overall dimensions, it is still necessary to protect the internal surface 1 of the insulation below the switch from the effects of condensation. In exposed hostile environments, this can only be done in a practical manner by filling the volume below the switch with a controlled environment such as dry nitrogen or SF6. This requires additional seals and monitoring and regular maintenance to ensure that the internal surface does not become contaminated. It is vitally important to ensure that the internal surface is kept clean and free from condensation and contamination, otherwise there is a risk of internal electrical discharge from the live conductor down the insulation to earth.
In FIG. 1, the current flowing through the device is carried by conductors 1, encapsulated in suitable electrically insulating material 2, such as epoxy resin or polymer concrete. Connection 9, flexible connection 4 and switch 7 provide the internal conducting path. Operation to open or close the switch 7 is performed by actuator 8 and lever 5. The integrity of the internal insulation surfaces is maintained by using SF6 gas or oil.
In FIG. 2, the current flowing through the device is similarly carried by conductors 21 and switch 22 through current transformer 23. In order to maintain the integrity of internal surface 24, some form of controlled environment is required, such as SF6 gas or dry nitrogen. Thus, in both cases it is necessary to protect the internal insulation surfaces by using a controlled environment, leading to additional costs and also risks of degradation and failure if the controlled environment is dissipated due to failure of seals and leakage.